In cold tandem rolling by emulsion lubrication, it is known that the emulsion lubrication oil supplied to the rolled material at the stand inlet side or the rolls separates into water and oil, the effect of the wedge shape formed at the roll bite inlet causes the oil to mainly be drawn into the roll bite due to its higher viscosity than the water, and therefore an oil film is formed between the rolls and the rolled material. Note that below, the phenomenon of the lubrication oil supplied to the rolls and the rolled material separating into water and oil and spreading is called “plate-out”.
In general, the effect of the wedge shape drawing in the lubrication oil is remarkably improved together with the increase in the rolling speed. Therefore, at a lower speed front stand, the friction coefficient becomes large, while at a higher speed rear stand, the friction coefficient becomes small. If the friction coefficient becomes larger, the possibility of burning flaws called “heat scratches” becomes higher. If the friction coefficient is too small, slip occurs and causes flaws. Therefore, with cold rolling, control of the friction coefficient to a suitable range becomes an important problem.
However, in a single rolling mill performing cold tandem rolling, usually one type of lubrication oil is used (for example, the base oil, emulsion concentration, temperature, etc. are managed to be constant). In the case of a rolling mill having two or more types of lubrication oil tanks, the lubrication base oil, the emulsion concentration, etc. can be changed. For example, a method of selective use of lubrication oils at a front stand and a rear stand etc. can be realized, so it is possible to advantageously control the friction coefficient in cold rolling to a suitable range.
In a rolling mill having only one tank, such selective use of lubrication oils is not possible. Further, newly increasing tanks would require capital investment, so while depending also on the kinds of the rolled products of the rolling mill, sometimes it is difficult to make full use of the capacity of the rolling mill with the current facilities as it and maintain the friction coefficients of all rolling stands in a suitable range for all sorts of rolled products.
Various inventions have been made up until now for solving such problems arising from lubrication of rolling. Note that increasing the friction coefficient can be relatively easily realized both technology and cost wise by decreasing the supply rate of the emulsion lubrication oil or decreasing the emulsion concentration, so in the past mainly methods for increasing the amount of plate-out to decrease the friction coefficient have been developed. Among these, as inventions for controlling the supply pressure etc. of nozzles to decrease the friction coefficient and thereby maintain the friction coefficient in a suitable range, there are the following examples. That is, Japanese Unexamined Patent Publication No. 7-009021 discloses an invention for adding a coagulating agent and setting the nozzle pressure to 5 kg/cm2 to 15 kg/cm2 (0.5 MPa to 1.5 MPa). Further, Japanese Unexamined Patent Publication No. 2001-269710 discloses an invention for setting the particle size of the emulsion and positions of the nozzles along with the nozzle pressure. These inventions, in a word, increase the nozzle pressure and increase the kinetic energy in order to increase the efficiency of friction of the lubrication oil to the rolled material. Further, they are based on the idea that since lubrication oil adhering to the rolled material separates into water and oil and is introduced into the roll bite, if the amount of plate-out to the rolled material increases, the amount of oil introduced also increases.